Astronomy

Traditionally, stars and planets are differentiated based on two properties:

(i) Whether or not they undergo nuclear reactions that burn hydrogen in their cores. Stars do this; planets don’t. In order to have high enough temperatures in the core to burn hydrogen, an object needs to have a mass of at least 75 or so times that of Jupiter. Anything more massive than that is automatically considered a star.

(ii) The way they form. Stars form when a cloud of gas, out in a nebula or other region of interstellar space, collapses under the influence of gravity. Planets, on the other hand, form when material in the disk around a pre-existing star begins to condense around rock/ice cores. We can have situations where the entire planet is almost completely rock/ice/water — called terrestrial planet (such as the Earth), or situations where a large amount of gas is subsequently attracted to the rock/ice core — called jovian planet (such as Jupiter, Saturn, etc.).

There is a transition area with objects that show some resemblance to a star and some to a planet. These objects are called brown dwarfs (we have discussed on it on several months ago). The mass of a brown dwarf lies between about 13-85 Mj (13 to 85 times the mass of Jupiter). They resemble jovian planets very well. The only difference between the two lies in there formation. As mentioned above, planets are normally formed out of the dust clouds surrounding the star. Where as brown dwarfs are formed by contraction of a gas cloud. It may be the case that Jupiter and Saturn are formed in the same way that stars are formed. In this case we can’t distinguish them from brown dwarfs.

Another important difference between the two is the small amount of nuclear fusion that can take place in the center of a brown dwarf. So this is maybe a better way to distinguish them from jovian planets. A problem here is that only the biggest brown dwarfs can sustain this fusion. The structure of brown dwarfs and jovian planets is very much the same. The chemical composition of these objects strongly depends on the temperature of the gasses in there atmospheres. Because jovian planets aren’t solid like terrestrial planets it is most likely that they can’t support life.

Because of the rapid progress there is made on this field of science, it won’t be long before we can say with more certainty what the atmospheres of these objects are made of and what there temperatures are. Maybe then it is possible to give a better definition of the difference between a star and a planet.

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